The myoelectric and mechanical responses of the human motor system to mechanical perturbations will be studied at three joints: ankle, elbow and wrist. Perturbations will be applied to subjects peforming a visual pursuit tracking task and controlling a dynamically difficult system in pursuit of the visual target. The measured responses to the perturbation will have both reflexive and voluntary components. We will make our measurements during the learning phase of skill acquisition in order to observe quantitative changes in those reflex and voluntary responses indicative of motor system adaptation during learning. The results will be informative both in terms of our understanding of motor learning mechanisms and of the underlying role for motor reflex mechanisms in voluntary motor control.